Integrated power stages, and in particular low-side and high-side driving stages, are formed by an integrated power transistor that delivers a current to an external load. These integrated power stages are used in numerous applications as a reliable and cost effective alternative to electro-mechanical relays.
While an integrated double-diffused MOS transistor (DMOS) represents an efficient device to switch relatively high powers, its structure must be carefully safeguarded by preventing damaging operating conditions which may cause the device to fail. Therefore, it is common practice to form special circuits that monitor operating conditions to avoid any accidental occurrence of these damaging conditions.
In particular, important information for correctly enabling switching of a low-side driver of the integrated DMOS transistor from a conduction state to an off state, or vice-versa, is one that excludes the existence of an open load condition. To provide this information, actuators in the form of integrated circuits are provided using low-side drivers having sensing circuits capable of timely signaling an eventual open load condition. Commonly, these circuits are required to detect whether an eventual open load state occurs during a phase in which the DMOS power transistor is off. This basic requirement of disabling the switching on of the transistor in case of an open load is currently provided by known circuits.
Although not generally required, sometimes it may be important to recognize an eventual open load condition also during a switched-on condition of the DMOS power transistor. For a low-side driver, it is difficult to satisfy this type of requirement when the current level of discrimination between a conduction state is established at a relatively low value. The conduction state must be defined and recognized as an open load condition. A low value is in the vicinity of 10-50 mA, for example.
The internal resistance Rdson of a conducting DMOS transistor is about 0.5 ohms, and often even lower. Consequently, a direct measurement of the current output by the transistor by monitoring the drain voltage becomes critical. By assuming a discriminating threshold of 30 mA and a resistance Rdson=0.5 oms, a drain-source voltage (i.e., across the transistor's current terminals) of V.sub.DS =15 mV would be available.